the following monsters fail!
any non humanoid non elemental monster that can cast magic. LOL@wolfs casting poison!!!!11
any monster that can revive heal or spawn more monsters
valkyrie profile 2 has all of the above :/
monsters that poison you suck too especially if the poison stays on your after battle hence making it far more lethal to you than it ever will be to a monster. resulting in psychological warfare.
moving on...theres this monster in shadowhearts 1 that i call the monster that slaps you with it's penis because thats what it does. it has a penis and slaps you multiple times with it. frankly i'm amazed something like that didnt get censored even though it was abstract. i also thought those garden soldiers you fight in FF8 were pretty cool. at least i dont have to question thier ability to use magic.